Using the compiler

Using the Commandline Compiler


This section does not apply to solcjs, not even if it is used in commandline mode.

One of the build targets of the Solidity repository is solc, the solidity commandline compiler. Using solc --help provides you with an explanation of all options. The compiler can produce various outputs, ranging from simple binaries and assembly over an abstract syntax tree (parse tree) to estimations of gas usage. If you only want to compile a single file, you run it as solc --bin sourceFile.sol and it will print the binary. If you want to get some of the more advanced output variants of solc, it is probably better to tell it to output everything to separate files using solc -o outputDirectory --bin --ast-json --asm sourceFile.sol.

Before you deploy your contract, activate the optimizer when compiling using solc --optimize --bin sourceFile.sol. By default, the optimizer will optimize the contract assuming it is called 200 times across its lifetime (more specifically, it assumes each opcode is executed around 200 times). If you want the initial contract deployment to be cheaper and the later function executions to be more expensive, set it to --optimize-runs=1. If you expect many transactions and do not care for higher deployment cost and output size, set --optimize-runs to a high number. This parameter has effects on the following (this might change in the future):

  • the size of the binary search in the function dispatch routine
  • the way constants like large numbers or strings are stored

The commandline compiler will automatically read imported files from the filesystem, but it is also possible to provide path redirects using prefix=path in the following way:

solc file.sol

This essentially instructs the compiler to search for anything starting with under /usr/local/lib/dapp-bin. solc will not read files from the filesystem that lie outside of the remapping targets and outside of the directories where explicitly specified source files reside, so things like import "/etc/passwd"; only work if you add /=/ as a remapping.

An empty remapping prefix is not allowed.

If there are multiple matches due to remappings, the one with the longest common prefix is selected.

When accessing the filesystem to search for imports, all paths are treated as if they were fully qualified paths. This behaviour can be customized by adding the command line option --base-path with a path to be prepended before each filesystem access for imports is performed. Furthermore, the part added via --base-path will not appear in the contract metadata.

For security reasons the compiler has restrictions what directories it can access. Paths (and their subdirectories) of source files specified on the commandline and paths defined by remappings are allowed for import statements, but everything else is rejected. Additional paths (and their subdirectories) can be allowed via the --allow-paths /sample/path,/another/sample/path switch.

Everything inside the path specified via --base-path is always allowed.

If your contracts use libraries, you will notice that the bytecode contains substrings of the form __$53aea86b7d70b31448b230b20ae141a537$__. These are placeholders for the actual library addresses. The placeholder is a 34 character prefix of the hex encoding of the keccak256 hash of the fully qualified library name. The bytecode file will also contain lines of the form // <placeholder> -> <fq library name> at the end to help identify which libraries the placeholders represent. Note that the fully qualified library name is the path of its source file and the library name separated by :. You can use solc as a linker meaning that it will insert the library addresses for you at those points:

Either add --libraries "file.sol:Math:0x1234567890123456789012345678901234567890 file.sol:Heap:0xabCD567890123456789012345678901234567890" to your command to provide an address for each library or store the string in a file (one library per line) and run solc using --libraries fileName.

If solc is called with the option --link, all input files are interpreted to be unlinked binaries (hex-encoded) in the __$53aea86b7d70b31448b230b20ae141a537$__-format given above and are linked in-place (if the input is read from stdin, it is written to stdout). All options except --libraries are ignored (including -o) in this case.

If solc is called with the option --standard-json, it will expect a JSON input (as explained below) on the standard input, and return a JSON output on the standard output. This is the recommended interface for more complex and especially automated uses. The process will always terminate in a “success” state and report any errors via the JSON output. The option --base-path is also processed in standard-json mode.


The library placeholder used to be the fully qualified name of the library itself instead of the hash of it. This format is still supported by solc --link but the compiler will no longer output it. This change was made to reduce the likelihood of a collision between libraries, since only the first 36 characters of the fully qualified library name could be used.

Setting the EVM version to target

When you compile your contract code you can specify the Ethereum virtual machine version to compile for to avoid particular features or behaviours.


Compiling for the wrong EVM version can result in wrong, strange and failing behaviour. Please ensure, especially if running a private chain, that you use matching EVM versions.

On the command line, you can select the EVM version as follows:

solc --evm-version <VERSION> contract.sol

In the standard JSON interface, use the "evmVersion" key in the "settings" field:

  "sources": { ... },
  "settings": {
    "optimizer": { ... },
    "evmVersion": "<VERSION>"

Target options

Below is a list of target EVM versions and the compiler-relevant changes introduced at each version. Backward compatibility is not guaranteed between each version.

  • homestead
    • (oldest version)
  • tangerineWhistle
    • Gas cost for access to other accounts increased, relevant for gas estimation and the optimizer.
    • All gas sent by default for external calls, previously a certain amount had to be retained.
  • spuriousDragon
    • Gas cost for the exp opcode increased, relevant for gas estimation and the optimizer.
  • byzantium
    • Opcodes returndatacopy, returndatasize and staticcall are available in assembly.
    • The staticcall opcode is used when calling non-library view or pure functions, which prevents the functions from modifying state at the EVM level, i.e., even applies when you use invalid type conversions.
    • It is possible to access dynamic data returned from function calls.
    • revert opcode introduced, which means that revert() will not waste gas.
  • constantinople
    • Opcodes create2`, ``extcodehash, shl, shr and sar are available in assembly.
    • Shifting operators use shifting opcodes and thus need less gas.
  • petersburg
    • The compiler behaves the same way as with constantinople.
  • istanbul (default)
    • Opcodes chainid and selfbalance are available in assembly.
  • berlin (experimental)

Compiler Input and Output JSON Description

The recommended way to interface with the Solidity compiler especially for more complex and automated setups is the so-called JSON-input-output interface. The same interface is provided by all distributions of the compiler.

The fields are generally subject to change, some are optional (as noted), but we try to only make backwards compatible changes.

The compiler API expects a JSON formatted input and outputs the compilation result in a JSON formatted output. The standard error output is not used and the process will always terminate in a “success” state, even if there were errors. Errors are always reported as part of the JSON output.

The following subsections describe the format through an example. Comments are of course not permitted and used here only for explanatory purposes.

Input Description

  // Required: Source code language. Currently supported are "Solidity" and "Yul".
  "language": "Solidity",
  // Required
    // The keys here are the "global" names of the source files,
    // imports can use other files via remappings (see below).
      // Optional: keccak256 hash of the source file
      // It is used to verify the retrieved content if imported via URLs.
      "keccak256": "0x123...",
      // Required (unless "content" is used, see below): URL(s) to the source file.
      // URL(s) should be imported in this order and the result checked against the
      // keccak256 hash (if available). If the hash doesn't match or none of the
      // URL(s) result in success, an error should be raised.
      // Using the commandline interface only filesystem paths are supported.
      // With the JavaScript interface the URL will be passed to the user-supplied
      // read callback, so any URL supported by the callback can be used.
        // If files are used, their directories should be added to the command line via
        // `--allow-paths <path>`.
      // Optional: keccak256 hash of the source file
      "keccak256": "0x234...",
      // Required (unless "urls" is used): literal contents of the source file
      "content": "contract destructible is owned { function shutdown() { if (msg.sender == owner) selfdestruct(owner); } }"
  // Optional
    // Optional: Sorted list of remappings
    "remappings": [ ":g=/dir" ],
    // Optional: Optimizer settings
    "optimizer": {
      // disabled by default
      "enabled": true,
      // Optimize for how many times you intend to run the code.
      // Lower values will optimize more for initial deployment cost, higher
      // values will optimize more for high-frequency usage.
      "runs": 200,
      // Switch optimizer components on or off in detail.
      // The "enabled" switch above provides two defaults which can be
      // tweaked here. If "details" is given, "enabled" can be omitted.
      "details": {
        // The peephole optimizer is always on if no details are given,
        // use details to switch it off.
        "peephole": true,
        // The unused jumpdest remover is always on if no details are given,
        // use details to switch it off.
        "jumpdestRemover": true,
        // Sometimes re-orders literals in commutative operations.
        "orderLiterals": false,
        // Removes duplicate code blocks
        "deduplicate": false,
        // Common subexpression elimination, this is the most complicated step but
        // can also provide the largest gain.
        "cse": false,
        // Optimize representation of literal numbers and strings in code.
        "constantOptimizer": false,
        // The new Yul optimizer. Mostly operates on the code of ABIEncoderV2
        // and inline assembly.
        // It is activated together with the global optimizer setting
        // and can be deactivated here.
        // Before Solidity 0.6.0 it had to be activated through this switch.
        "yul": false,
        // Tuning options for the Yul optimizer.
        "yulDetails": {
          // Improve allocation of stack slots for variables, can free up stack slots early.
          // Activated by default if the Yul optimizer is activated.
          "stackAllocation": true,
          // Select optimization steps to be applied.
          // Optional, the optimizer will use the default sequence if omitted.
          "optimizerSteps": "dhfoDgvulfnTUtnIf..."
    // Version of the EVM to compile for.
    // Affects type checking and code generation. Can be homestead,
    // tangerineWhistle, spuriousDragon, byzantium, constantinople, petersburg, istanbul or berlin
    "evmVersion": "byzantium",
    // Optional: Debugging settings
    "debug": {
      // How to treat revert (and require) reason strings. Settings are
      // "default", "strip", "debug" and "verboseDebug".
      // "default" does not inject compiler-generated revert strings and keeps user-supplied ones.
      // "strip" removes all revert strings (if possible, i.e. if literals are used) keeping side-effects
      // "debug" injects strings for compiler-generated internal reverts, implemented for ABI encoders V1 and V2 for now.
      // "verboseDebug" even appends further information to user-supplied revert strings (not yet implemented)
      "revertStrings": "default"
    // Metadata settings (optional)
    "metadata": {
      // Use only literal content and not URLs (false by default)
      "useLiteralContent": true,
      // Use the given hash method for the metadata hash that is appended to the bytecode.
      // The metadata hash can be removed from the bytecode via option "none".
      // The other options are "ipfs" and "bzzr1".
      // If the option is omitted, "ipfs" is used by default.
      "bytecodeHash": "ipfs"
    // Addresses of the libraries. If not all libraries are given here,
    // it can result in unlinked objects whose output data is different.
    "libraries": {
      // The top level key is the the name of the source file where the library is used.
      // If remappings are used, this source file should match the global path
      // after remappings were applied.
      // If this key is an empty string, that refers to a global level.
      "myFile.sol": {
        "MyLib": "0x123123..."
    // The following can be used to select desired outputs based
    // on file and contract names.
    // If this field is omitted, then the compiler loads and does type checking,
    // but will not generate any outputs apart from errors.
    // The first level key is the file name and the second level key is the contract name.
    // An empty contract name is used for outputs that are not tied to a contract
    // but to the whole source file like the AST.
    // A star as contract name refers to all contracts in the file.
    // Similarly, a star as a file name matches all files.
    // To select all outputs the compiler can possibly generate, use
    // "outputSelection: { "*": { "*": [ "*" ], "": [ "*" ] } }"
    // but note that this might slow down the compilation process needlessly.
    // The available output types are as follows:
    // File level (needs empty string as contract name):
    //   ast - AST of all source files
    //   legacyAST - legacy AST of all source files
    // Contract level (needs the contract name or "*"):
    //   abi - ABI
    //   devdoc - Developer documentation (natspec)
    //   userdoc - User documentation (natspec)
    //   metadata - Metadata
    //   ir - Yul intermediate representation of the code before optimization
    //   irOptimized - Intermediate representation after optimization
    //   storageLayout - Slots, offsets and types of the contract's state variables.
    //   evm.assembly - New assembly format
    //   evm.legacyAssembly - Old-style assembly format in JSON
    //   evm.bytecode.object - Bytecode object
    //   evm.bytecode.opcodes - Opcodes list
    //   evm.bytecode.sourceMap - Source mapping (useful for debugging)
    //   evm.bytecode.linkReferences - Link references (if unlinked object)
    //   evm.deployedBytecode* - Deployed bytecode (has all the options that evm.bytecode has)
    //   evm.deployedBytecode.immutableReferences - Map from AST ids to bytecode ranges that reference immutables
    //   evm.methodIdentifiers - The list of function hashes
    //   evm.gasEstimates - Function gas estimates
    //   ewasm.wast - eWASM S-expressions format (not supported at the moment)
    //   ewasm.wasm - eWASM binary format (not supported at the moment)
    // Note that using a using `evm`, `evm.bytecode`, `ewasm`, etc. will select every
    // target part of that output. Additionally, `*` can be used as a wildcard to request everything.
    "outputSelection": {
      "*": {
        "*": [
          "metadata", "evm.bytecode" // Enable the metadata and bytecode outputs of every single contract.
          , "evm.bytecode.sourceMap" // Enable the source map output of every single contract.
        "": [
          "ast" // Enable the AST output of every single file.
      // Enable the abi and opcodes output of MyContract defined in file def.
      "def": {
        "MyContract": [ "abi", "evm.bytecode.opcodes" ]

Output Description

  // Optional: not present if no errors/warnings were encountered
  "errors": [
      // Optional: Location within the source file.
      "sourceLocation": {
        "file": "sourceFile.sol",
        "start": 0,
        "end": 100
      // Optional: Further locations (e.g. places of conflicting declarations)
      "secondarySourceLocations": [
          "file": "sourceFile.sol",
          "start": 64,
          "end": 92,
          "message": "Other declaration is here:"
      // Mandatory: Error type, such as "TypeError", "InternalCompilerError", "Exception", etc.
      // See below for complete list of types.
      "type": "TypeError",
      // Mandatory: Component where the error originated, such as "general", "ewasm", etc.
      "component": "general",
      // Mandatory ("error" or "warning")
      "severity": "error",
      // Optional: unique code for the cause of the error
      "errorCode": "3141",
      // Mandatory
      "message": "Invalid keyword",
      // Optional: the message formatted with source location
      "formattedMessage": "sourceFile.sol:100: Invalid keyword"
  // This contains the file-level outputs.
  // It can be limited/filtered by the outputSelection settings.
  "sources": {
    "sourceFile.sol": {
      // Identifier of the source (used in source maps)
      "id": 1,
      // The AST object
      "ast": {},
      // The legacy AST object
      "legacyAST": {}
  // This contains the contract-level outputs.
  // It can be limited/filtered by the outputSelection settings.
  "contracts": {
    "sourceFile.sol": {
      // If the language used has no contract names, this field should equal to an empty string.
      "ContractName": {
        // The Ethereum Contract ABI. If empty, it is represented as an empty array.
        // See
        "abi": [],
        // See the Metadata Output documentation (serialised JSON string)
        "metadata": "{...}",
        // User documentation (natspec)
        "userdoc": {},
        // Developer documentation (natspec)
        "devdoc": {},
        // Intermediate representation (string)
        "ir": "",
        // See the Storage Layout documentation.
        "storageLayout": {"storage": [...], "types": {...} },
        // EVM-related outputs
        "evm": {
          // Assembly (string)
          "assembly": "",
          // Old-style assembly (object)
          "legacyAssembly": {},
          // Bytecode and related details.
          "bytecode": {
            // The bytecode as a hex string.
            "object": "00fe",
            // Opcodes list (string)
            "opcodes": "",
            // The source mapping as a string. See the source mapping definition.
            "sourceMap": "",
            // If given, this is an unlinked object.
            "linkReferences": {
              "libraryFile.sol": {
                // Byte offsets into the bytecode.
                // Linking replaces the 20 bytes located there.
                "Library1": [
                  { "start": 0, "length": 20 },
                  { "start": 200, "length": 20 }
          "deployedBytecode": {
            ..., // The same layout as above.
            "immutableReferences": [
              // There are two references to the immutable with AST ID 3, both 32 bytes long. One is
              // at bytecode offset 42, the other at bytecode offset 80.
              "3": [{ "start": 42, "length": 32 }, { "start": 80, "length": 32 }]
          // The list of function hashes
          "methodIdentifiers": {
            "delegate(address)": "5c19a95c"
          // Function gas estimates
          "gasEstimates": {
            "creation": {
              "codeDepositCost": "420000",
              "executionCost": "infinite",
              "totalCost": "infinite"
            "external": {
              "delegate(address)": "25000"
            "internal": {
              "heavyLifting()": "infinite"
        // eWASM related outputs
        "ewasm": {
          // S-expressions format
          "wast": "",
          // Binary format (hex string)
          "wasm": ""

Error types

  1. JSONError: JSON input doesn’t conform to the required format, e.g. input is not a JSON object, the language is not supported, etc.
  2. IOError: IO and import processing errors, such as unresolvable URL or hash mismatch in supplied sources.
  3. ParserError: Source code doesn’t conform to the language rules.
  4. DocstringParsingError: The NatSpec tags in the comment block cannot be parsed.
  5. SyntaxError: Syntactical error, such as continue is used outside of a for loop.
  6. DeclarationError: Invalid, unresolvable or clashing identifier names. e.g. Identifier not found
  7. TypeError: Error within the type system, such as invalid type conversions, invalid assignments, etc.
  8. UnimplementedFeatureError: Feature is not supported by the compiler, but is expected to be supported in future versions.
  9. InternalCompilerError: Internal bug triggered in the compiler - this should be reported as an issue.
  10. Exception: Unknown failure during compilation - this should be reported as an issue.
  11. CompilerError: Invalid use of the compiler stack - this should be reported as an issue.
  12. FatalError: Fatal error not processed correctly - this should be reported as an issue.
  13. Warning: A warning, which didn’t stop the compilation, but should be addressed if possible.

Compiler tools


solidity-upgrade can help you to semi-automatically upgrade your contracts to breaking language changes. While it does not and cannot implement all required changes for every breaking release, it still supports the ones, that would need plenty of repetitive manual adjustments otherwise.


solidity-upgrade carries out a large part of the work, but your contracts will most likely need further manual adjustments. We recommend using a version control system for your files. This helps reviewing and eventually rolling back the changes made.


solidity-upgrade is not considered to be complete or free from bugs, so please use with care.

How it works

You can pass (a) Solidity source file(s) to solidity-upgrade [files]. If these make use of import statement which refer to files outside the current source file’s directory, you need to specify directories that are allowed to read and import files from, by passing --allow-paths [directory]. You can ignore missing files by passing --ignore-missing.

solidity-upgrade is based on libsolidity and can parse, compile and analyse your source files, and might find applicable source upgrades in them.

Source upgrades are considered to be small textual changes to your source code. They are applied to an in-memory representation of the source files given. The corresponding source file is updated by default, but you can pass --dry-run to simulate to whole upgrade process without writing to any file.

The upgrade process itself has two phases. In the first phase source files are parsed, and since it is not possible to upgrade source code on that level, errors are collected and can be logged by passing --verbose. No source upgrades available at this point.

In the second phase, all sources are compiled and all activated upgrade analysis modules are run alongside compilation. By default, all available modules are activated. Please read the documentation on available modules for further details.

This can result in compilation errors that may be fixed by source upgrades. If no errors occur, no source upgrades are being reported and you’re done. If errors occur and some upgrade module reported a source upgrade, the first reported one gets applied and compilation is triggered again for all given source files. The previous step is repeated as long as source upgrades are reported. If errors still occur, you can log them by passing --verbose. If no errors occur, your contracts are up to date and can be compiled with the latest version of the compiler.

Available upgrade modules

Module Version Description
constructor 0.5.0 Constructors must now be defined using the constructor keyword.
visibility 0.5.0 Explicit function visibility is now mandatory, defaults to public.
abstract 0.6.0 The keyword abstract has to be used if a contract does not implement all its functions.
virtual 0.6.0 Functions without implementation outside an interface have to be marked virtual.
override 0.6.0 When overriding a function or modifier, the new keyword override must be used.
dotsyntax 0.7.0 The following syntax is deprecated: f.gas(...)(), f.value(...)() and (new C).value(...)(). Replace these calls by f{gas: ..., value: ...}() and (new C){value: ...}().
now 0.7.0 The now keyword is deprecated. Use block.timestamp instead.
constructor-visibility 0.7.0 Removes visibility of constructors.

Please read 0.5.0 release notes, 0.6.0 release notes and 0.7.0 release notes for further details.


Usage: solidity-upgrade [options] contract.sol

Allowed options:
    --help               Show help message and exit.
    --version            Show version and exit.
    --allow-paths path(s)
                         Allow a given path for imports. A list of paths can be
                         supplied by separating them with a comma.
    --ignore-missing     Ignore missing files.
    --modules module(s)  Only activate a specific upgrade module. A list of
                         modules can be supplied by separating them with a comma.
    --dry-run            Apply changes in-memory only and don't write to input
    --verbose            Print logs, errors and changes. Shortens output of
                         upgrade patches.
    --unsafe             Accept *unsafe* changes.

Bug Reports / Feature requests

If you found a bug or if you have a feature request, please file an issue on Github.


Assume that you have the following contract in Source.sol:

pragma solidity >=0.6.0 <0.6.4;
// This will not compile after 0.7.0
// SPDX-License-Identifier: GPL-3.0
contract C {
    // FIXME: remove constructor visibility and make the contract abstract
    constructor() internal {}

contract D {
    uint time;

    function f() public payable {
        // FIXME: change now to block.timestamp
        time = now;

contract E {
    D d;

    // FIXME: remove constructor visibility
    constructor() public {}

    function g() public {
        // FIXME: change .value(5) =>  {value: 5}
Required changes

The above contract will not compile starting from 0.7.0. To bring the contract up to date with the current Solidity version, the following upgrade modules have to be executed: constructor-visibility, now and dotsyntax. Please read the documentation on available modules for further details.

Running the upgrade

It is recommended to explicitly specify the upgrade modules by using --modules argument.

$ solidity-upgrade --modules constructor-visibility,now,dotsyntax Source.sol

The command above applies all changes as shown below. Please review them carefully (the pragmas will have to be updated manually.)

pragma solidity >0.6.99 <0.8.0;
// SPDX-License-Identifier: GPL-3.0
abstract contract C {
    // FIXME: remove constructor visibility and make the contract abstract
    constructor() {}

contract D {
    uint time;

    function f() public payable {
        // FIXME: change now to block.timestamp
        time = block.timestamp;

contract E {
    D d;

    // FIXME: remove constructor visibility
    constructor() {}

    function g() public {
        // FIXME: change .value(5) =>  {value: 5}
        d.f{value: 5}();